Conventionally, two types of motors, induction motor and synchronous motor, have been used as an alternating current motor which is a power source for a machine tool or the like. An induction motor has a configuration in which an induced current occurs in a rotor due to a revolving magnetic field generated by a stator and thereby a rotational torque corresponding to slip is generated, while a synchronous motor has a configuration in which a rotor rotates following a surrounding revolving magnetic field generated by an applied alternating current.
The advantages of an induction motor include that it has a simple configuration, that it is not easily broken because of its rigidity, that the efficiency in a high speed rotation region can be improved by the field weakening control, and that it is inexpensive. On the other hand, the disadvantages of an induction motor include that, because of the mechanism thereof, the frequency response band cannot be set to be high as compared with a synchronous motor, that heat generation in the rotor is large for causing a secondary induced current to flow, and that the torque ripple is large.
In contrast, the advantages of a synchronous motor include that the frequency response band can be set to be high, that heat generation in the rotor is small because of no secondary induced current, that it has a high efficiency because of a large torque in a low speed region, and that it is easily made compact in size as compared with an induction motor. On the other hand, the disadvantages of a synchronous motor include that it is often expensive because of use of a magnet, and that iron loss in a high speed rotation region is large because of the magnetic force of the magnet.
In the field of machine tool, mainly, an induction motor is widely used as a driving power source, and the reasons therefor are that the above-mentioned advantages of the induction motor are preferred, and that emphasis is placed on the fact that the induction motor can withstand an impact because of the presence of the slip phenomenon in which a torque occurs late when external disturbance occurs.
However, even in the field of machine tool, depending on cutting conditions, a purpose of cutting, a vibration system model of device structure and conditions of workpiece to be machined, there is the case where a synchronous motor is more advantageous than an induction motor. For example, when emphasis is placed on improving the rigidity and the stability of the motor in accordance with a purpose of cutting or the like, it is advantageous to employ a synchronous motor instead of an induction motor because it is preferable that the frequency response band of the motor is high.
Thus, because an induction motor and a synchronous motor are different from each other in their properties and their advantages and disadvantages, it would be convenient if it is possible to switch between them flexibly in accordance with a situation and a purpose.
In the Patent Literature 1, an induction synchronous motor in which an IM rotor rotating as induction motor and a PM rotor rotating as synchronous motor are connected to each other by an output shaft is suggested. In this induction synchronous motor, a composite current is generated by superimposing a current for induction motor for rotating the IM rotor and a current for synchronous motor for rotating the PM rotor, and this composite current is caused to flow through a coil of a stator. With this induction synchronous motor, it is possible to operate a single alternating current motor both as induction motor and as synchronous motor, and therefore it is possible to obtain driving characteristics of both motors depending on circumstances and in accordance with a situation and a purpose without preparing a plurality of motors.